The invention relates to a method for weighing sample tubes which are delivered in a sample tube rack, as used especially for preparing for centrifuging samples in chemical, biological and medical laboratories, and a workstation as used in association with such and similar methods.
In the laboratory, it is often necessary to determine individually the weights of sample tubes, which are usually delivered in the form of several sample tubes in a sample tube rack. If, for example, the liquid samples are to be centrifuged, the sample tubes must be distributed over a plurality of sample tube buckets, usually four sample tube buckets, which are then introduced into the centrifuge. To limit the imbalance, the weights of sample tube buckets opposite one another may differ from one another by not more than a maximum value of, usually, between 15 g and 20 g.
WO-A-98/01 760 discloses a method of the generic type and a workstation of the generic type for carrying out said method. According to this publication, the weights of sample tubes are determined by removing them in succession from a sample tube rack and placing them by means of a transfer device in a sample tube bucket which is arranged on a balance. The weight of each sample tube is determined from the increase in the weight of the sample tube bucket caused by adding a sample tube. The sample tubes are distributed over sample tube buckets in such a way that the latter form pairs so that the total weights of the sample tube buckets belonging to one pair differ by not more than 10 g. In this method, the weight is not determined until after the addition of the sample tube, so that the weight determined cannot be used for controlling the addition of the sample tubes themselves, in particular their distribution over the sample tube buckets, according to the determined weights and for avoiding incorrect distributions from the outset. Errors have to be corrected subsequently by redistributing the sample tubes or by using balancing weights, which can result in considerable time losses.
GB-A-997 226 discloses a transport track having conveyor belts, with a balance which has a platform which can be raised and by means of which articles can be lifted off the transport track and their weight determined, whereupon they are set down again on the transport track by lowering the platform and are transported further. The device is not suitable for removing parts of articles and determining the weights of these parts.
U.S. Pat. No. 3,489,521 describes a workstation having a feed device for sample tube racks, comprising a feed track and a return track parallel to said feed track, both having conveyor belts, and comprising a transverse intermediate track which connects the feed track to the return track. It also has a centrifuge in which the sample tubes are centrifuged without prior weighing in the sample tube racks.
It is the object of the invention to simplify and to accelerate the weighing of the sample tubes in comparison with the known methods of the generic type. This object is achieved by the features in the characterizing clause of claim 1. In the method according to the invention, intermediate deposition of the sample tube whose weight is to be determined is unnecessary. It can be removed from the sample tube rack and brought directly, for example, to a sample tube bucket and deposited there. The determination of the weight causes no delay.
Furthermore, the workstation of the generic type is to be adapted in such a way that it is suitable for carrying out the method according to the invention and in particular substantially facilitates the feeding and weighing of sample tubes. This object is achieved by the features in the characterizing clause of claim 4. The workstation according to the invention permits simple feeding of the sample tube racks to the balance and convenient removal of individual sample tubes therefrom.